Metrology method and apparatus and associated computer product

ABSTRACT

Disclosed is a process monitoring method, and an associated metrology apparatus. The method comprises: obtaining measured target response sequence data relating to a measurement response of a target formed on a substrate by a lithographic process to measurement radiation comprising multiple measurement profiles, wherein the measured target response sequence data describes a variation of the measurement response of the target in response to variations of the measurement profiles; obtaining reference target response sequence data relating to a measurement response of the target as designed to the measurement radiation, wherein the reference target response sequence data describes an optimal measurement response of the target in response to designed measurement profiles without un-designed variation; comparing the measured target response sequence data and the reference target response sequence data; and determining values for variations in stack parameters of the target from the measured target response sequence data based on the comparison.

This application is a continuation of U.S. patent application Ser. No.15/874,972, filed Jan. 19, 2018, now U.S. Pat. No. 10,310,388, whichclaims benefit of U.S. Provisional Application No. 62/453,743, filedFeb. 2, 2017, and is incorporated herein in its entirety by reference.

FIELD

The present invention relates to a method, apparatus, and computerproduct for metrology usable, for example, in the manufacture of devicesby a lithographic technique and to a method of manufacturing devicesusing a lithographic technique.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned.

In a lithographic process (i.e., a process of developing a device orother structure involving lithographic exposure, which may typicallyinclude one or more associated processing steps such as development ofresist, etching, etc.), it is desirable frequently to make measurementsof structures created, e.g., for process control and verification.Various tools for making such measurements are known, including scanningelectron microscopes, which are often used to measure critical dimension(CD), and specialized tools to measure overlay, the accuracy ofalignment of two layers of a substrate. Recently, various forms ofscatterometers have been developed for use in the lithographic field.These devices direct a beam of radiation onto a target and measure oneor more properties of the scattered radiation—e.g., intensity at asingle angle of reflection as a function of wavelength; intensity at oneor more wavelengths as a function of reflected angle; or polarization asa function of reflected angle—to obtain a “spectrum” from which aproperty of interest of the target can be determined. Determination ofthe property of interest may be performed by various techniques: e.g.,reconstruction of the target structure by iterative approaches such asrigorous coupled wave analysis or finite element methods; librarysearches; and principal component analysis.

SUMMARY

The accuracy of target measurements depends on the combination ofcertain target or stack parameters and characteristics of themeasurement radiation (the measurement profile) used. Therefore, targetparameter and measurement profile optimization may be performed tooptimize accuracy of target measurements. However, if the stackparameters of the target vary from those of targets used in theoptimization step, then the measurement profile may no longer be optimalfor the target during a measurement.

In addition, it may be desirable to monitor certain stack parameters ofa target, for example, the heights of some or all of the layers within astack. Measurement of layer heights is usually performed on dedicatedthin film targets, separate to overlay and alignment targets. These takeup additional substrate area, and their measurement takes additionalmeasurement time.

In a first aspect of the invention there is provided a method ofmetrology in a metrology apparatus for monitoring a measurement process,comprising: obtaining measured target response sequence data relating toa measurement response of one or more targets formed on a substrate by alithographic process to measurement radiation comprising a plurality ofmeasurement profiles, wherein the measured target response sequence datadescribes a variation of the measurement response of the one or moretargets in response to variations of the plurality of measurementprofiles; obtaining reference target response sequence data relating toa measurement response of the one or more targets as designed to themeasurement radiation, wherein the reference target response sequencedata describes an optimal measurement response of the one or moretargets in response to a designed plurality of measurement profileswithout un-designed variation; comparing the measured target responsesequence data and the reference target response sequence data; anddetermining values for variations in one or more stack parameters of theone or more targets from the measured target response sequence databased on the comparison.

In a second aspect of the invention there is provided a metrologyapparatus comprising: an illumination system configured to illuminateone or more targets formed on a substrate by a lithographic process withmeasurement radiation comprising a plurality of measurement profiles; adetection system configured to detect scattered radiation arising fromillumination of said one or more targets; and a processor operable to:derive measured target response sequence data from the detectedscattered radiation, wherein the measured target response sequence datadescribes a variation of a measurement response of the one or moretargets in response to variations of the plurality of measurementprofiles, compare the measured target response sequence data toreference target response sequence data relating to a measurementresponse of the one or more targets as designed to the measurementradiation, wherein the reference target response sequence data describesan optimal measurement response of the one or more targets in responseto a designed plurality of measurement profiles without un-designedvariation, and determine values for variations in one or more stackparameters of the one or more targets from the measured target responsesequence data based on the comparison.

Another aspect of the invention comprises a computer program andassociated computer program carrier for performing the method of thefirst aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which.

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a lithographic cell or cluster according to an embodimentof the invention;

FIG. 3(a) is schematic diagram of a dark field measurement apparatus foruse in measuring targets according to embodiments of the invention usinga first pair of illumination apertures providing certain illuminationmodes;

FIG. 3(b) is a schematic detail of a diffraction spectrum of a targetfor a given direction of illumination;

FIG. 3(c) is a schematic illustration of a second pair of illuminationapertures providing further illumination modes in using a measurementapparatus for diffraction based overlay measurements;

FIG. 3(d) is a schematic illustration of a third pair of illuminationapertures combining the first and second pairs of apertures providingfurther illumination modes in using a measurement apparatus fordiffraction based overlay measurements;

FIG. 4 depicts a form of multiple periodic structure (e.g., multiplegrating) target and an outline of a measurement spot on a substrate;

FIG. 5 depicts an image of the target of FIG. 4 obtained in theapparatus of FIG. 3;

FIG. 6 is a flowchart showing the steps of an overlay measurement methodusing the apparatus of FIG. 3 and adaptable to embodiments of thepresent invention;

FIGS. 7(a) to 7(d) show schematic cross-sections of overlay periodicstructures (e.g., gratings) having different overlay values in theregion of zero;

FIG. 8 illustrates principles of overlay measurement in an ideal targetstructure;

FIG. 9 is a graph of overlay sensitivity K against wavelength λ(nm) fora target, also referred to as a swing curve; and

FIG. 10 is a flowchart describing a method according to a firstembodiment of the invention.

DETAILED DESCRIPTION

Before describing embodiments in detail, it is instructive to present anexample environment in which embodiments may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a patterningdevice support or support structure (e.g., a mask table) MT constructedto support a patterning device (e.g., a mask) MA and connected to afirst positioner PM configured to accurately position the patterningdevice in accordance with certain parameters; a substrate table (e.g., awafer table) WT constructed to hold a substrate (e.g., a resist coatedwafer) W and connected to a second positioner μW configured toaccurately position the substrate in accordance with certain parameters;and a projection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., including one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner μW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M₁, M₂ and substrate alignment marks P₁, P₂.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. An embodiment of an alignmentsystem, which can detect the alignment markers, is described furtherbelow.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support (e.g., mask table) MT andthe substrate table WTa are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e., a single static exposure). The substratetable WTa is then shifted in the X and/or Y direction so that adifferent target portion C can be exposed. In step mode, the maximumsize of the exposure field limits the size of the target portion Cimaged in a single static exposure.

2. In scan mode, the patterning device support (e.g., mask table) MT andthe substrate table WTa are scanned synchronously while a patternimparted to the radiation beam is projected onto a target portion C(i.e., a single dynamic exposure). The velocity and direction of thesubstrate table WTa relative to the patterning device support (e.g.,mask table) MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the patterning device support (e.g., mask table) MTis kept essentially stationary holding a programmable patterning device,and the substrate table WTa is moved or scanned while a pattern impartedto the radiation beam is projected onto a target portion C. In thismode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after eachmovement of the substrate table WTa or in between successive radiationpulses during a scan. This mode of operation can be readily applied tomaskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Lithographic apparatus LA is of a so-called dual stage type which hastwo tables WTa, WTb (e.g., two substrate tables) and two stations—anexposure station and a measurement station—between which the tables canbe exchanged. For example, while a substrate on one table is beingexposed at the exposure station, another substrate can be loaded ontothe other substrate table at the measurement station and variouspreparatory steps carried out. The preparatory steps may include mappingthe surface control of the substrate using a level sensor LS andmeasuring the position of alignment markers on the substrate using analignment sensor AS, both sensors being supported by a reference frameRF. If the position sensor IF is not capable of measuring the positionof a table while it is at the measurement station as well as at theexposure station, a second position sensor may be provided to enable thepositions of the table to be tracked at both stations. As anotherexample, while a substrate on one table is being exposed at the exposurestation, another table without a substrate waits at the measurementstation (where optionally measurement activity may occur). This othertable has one or more measurement devices and may optionally have othertools (e.g., cleaning apparatus). When the substrate has completedexposure, the table without a substrate moves to the exposure station toperform, e.g., measurements and the table with the substrate moves to alocation (e.g., the measurement station) where the substrate is unloadedand another substrate is load. These multi-table arrangements enable asubstantial increase in the throughput of the apparatus.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to as a lithocell orlithocluster, which also includes apparatus to perform one or more pre-and post-exposure processes on a substrate. Conventionally these includeone or more spin coaters SC to deposit a resist layer, one or moredevelopers DE to develop exposed resist, one or more chill plates CH andone or more bake plates BK. A substrate handler, or robot, RO picks up asubstrate from input/output ports I/O1, I/O2, moves it between thedifferent process devices and delivers it to the loading bay LB of thelithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithographic controlunit LACU. Thus, the different apparatus may be operated to maximizethroughput and processing efficiency.

In order that the substrate that is exposed by the lithographicapparatus is exposed correctly and consistently, it is desirable toinspect an exposed substrate to measure one or more properties such asoverlay error between subsequent layers, line thickness, criticaldimension (CD), etc. If an error is detected, an adjustment may be madeto an exposure of one or more subsequent substrates, especially if theinspection can be done soon and fast enough that another substrate ofthe same batch is still to be exposed. Also, an already exposedsubstrate may be stripped and reworked (to improve yield) or discarded,thereby avoiding performing an exposure on a substrate that is known tobe faulty. In a case where only some target portions of a substrate arefaulty, a further exposure may be performed only on those targetportions which are good. Another possibility is to adapt a setting of asubsequent process step to compensate for the error, e.g. the time of atrim etch step can be adjusted to compensate for substrate-to-substrateCD variation resulting from the lithographic process step.

An inspection apparatus is used to determine one or more properties of asubstrate, and in particular, how one or more properties of differentsubstrates or different layers of the same substrate vary from layer tolayer and/or across a substrate. The inspection apparatus may beintegrated into the lithographic apparatus LA or the lithocell LC or maybe a stand-alone device. To enable most rapid measurements, it isdesirable that the inspection apparatus measure one or more propertiesin the exposed resist layer immediately after the exposure. However, thelatent image in the resist has a very low contrast—there is only a verysmall difference in refractive index between the part of the resistwhich has been exposed to radiation and that which has not—and not allinspection apparatus have sufficient sensitivity to make usefulmeasurements of the latent image. Therefore measurements may be takenafter the post-exposure bake step (PEB) which is customarily the firststep carried out on an exposed substrate and increases the contrastbetween exposed and unexposed parts of the resist. At this stage, theimage in the resist may be referred to as semi-latent. It is alsopossible to make measurements of the developed resist image—at whichpoint either the exposed or unexposed parts of the resist have beenremoved—or after a pattern transfer step such as etching. The latterpossibility limits the possibility for rework of a faulty substrate butmay still provide useful information, e.g. for the purpose of processcontrol.

A target used by a conventional scatterometer comprises a relativelylarge periodic structure layout (e.g., comprising one or more gratings),e.g., 40 μm by 40 μm. In that case, the measurement beam often has aspot size that is smaller than the periodic structure layout (i.e., thelayout is underfilled such that one or more of the periodic structuresis not completely covered by the spot). This simplifies mathematicalreconstruction of the target as it can be regarded as infinite. However,for example, so the target can be positioned in among product features,rather than in the scribe lane, the size of a target has been reduced,e.g., to 20 μm by 20 μm or less, or to 10 μm by 10 μm or less. In thissituation, the periodic structure layout may be made smaller than themeasurement spot (i.e., the periodic structure layout is overfilled).Typically such a target is measured using dark field scatterometry inwhich the zeroth order of diffraction (corresponding to a specularreflection) is blocked, and only higher orders processed. Examples ofdark field metrology can be found in PCT patent application publicationnos. WO 2009/078708 and WO 2009/106279, which are hereby incorporated intheir entirety by reference. Further developments of the technique havebeen described in U.S. patent application publications US2011-0027704,US2011-0043791 and US2012-0242970, which are hereby incorporated intheir entirety by reference. Diffraction-based overlay (DBO or μDBO)using dark-field detection of the diffraction orders enables overlaymeasurements on smaller targets. These targets can be smaller than theillumination spot and may be surrounded by product structures on asubstrate. In an embodiment, multiple targets can be measured in oneimage.

In an embodiment, the target on a substrate may comprise one or more 1-Dperiodic gratings, which are printed such that after development, thebars are formed of solid resist lines. In an embodiment, the target maycomprise one or more 2-D periodic gratings, which are printed such thatafter development, the one or more gratings are formed of solid resistpillars or vias in the resist. The bars, pillars or vias mayalternatively be etched into the substrate. The pattern of the gratingis sensitive to chromatic aberrations in the lithographic projectionapparatus, particularly the projection system PL, and illuminationsymmetry and the presence of such aberrations will manifest themselvesin a variation in the printed grating. Accordingly, the measured data ofthe printed gratings can be used to reconstruct the gratings. Theparameters of the 1-D grating, such as line widths and shapes, orparameters of the 2-D grating, such as pillar or via widths or lengthsor shapes, may be input to the reconstruction process, performed byprocessing unit PU, from knowledge of the printing step and/or othermeasurement processes.

A dark field metrology apparatus suitable for use in embodiments of theinvention is shown in FIG. 3(a). A target T (comprising a periodicstructure such as a grating) and diffracted rays are illustrated in moredetail in FIG. 3(b). The dark field metrology apparatus may be astand-alone device or incorporated in either the lithographic apparatusLA, e.g., at the measurement station, or the lithographic cell LC. Anoptical axis, which has several branches throughout the apparatus, isrepresented by a dotted line O. In this apparatus, radiation emitted byan output 11 (e.g., a source such as a laser or a xenon lamp or anopening connected to a source) is directed onto substrate W via a prism15 by an optical system comprising lenses 12, 14 and objective lens 16.These lenses are arranged in a double sequence of a 4F arrangement. Adifferent lens arrangement can be used, provided that it still providesa substrate image onto a detector.

In an embodiment, the lens arrangement allows for access of anintermediate pupil-plane for spatial-frequency filtering. Therefore, theangular range at which the radiation is incident on the substrate can beselected by defining a spatial intensity distribution in a plane thatpresents the spatial spectrum of the substrate plane, here referred toas a (conjugate) pupil plane. In particular, this can be done, forexample, by inserting an aperture plate 13 of suitable form betweenlenses 12 and 14, in a plane which is a back-projected image of theobjective lens pupil plane. In the example illustrated, aperture plate13 has different forms, labeled 13N and 13S, allowing differentillumination modes to be selected. The illumination system in thepresent examples forms an off-axis illumination mode. In the firstillumination mode, aperture plate 13N provides off-axis illuminationfrom a direction designated, for the sake of description only, as‘north’. In a second illumination mode, aperture plate 13S is used toprovide similar illumination, but from an opposite direction, labeled‘south’. Other modes of illumination are possible by using differentapertures. The rest of the pupil plane is desirably dark as anyunnecessary radiation outside the desired illumination mode mayinterfere with the desired measurement signals.

As shown in FIG. 3(b), target T is placed with substrate W substantiallynormal to the optical axis O of objective lens 16. A ray of illuminationI impinging on target T from an angle off the axis O gives rise to azeroth order ray (solid line O) and two first order rays (dot-chain line+1 and double dot-chain line −1). With an overfilled small target T,these rays are just one of many parallel rays covering the area of thesubstrate including metrology target T and other features. Since theaperture in plate 13 has a finite width (necessary to admit a usefulquantity of radiation), the incident rays I will in fact occupy a rangeof angles, and the diffracted rays 0 and +1/−1 will be spread outsomewhat. According to the point spread function of a small target, eachorder +1 and −1 will be further spread over a range of angles, not asingle ideal ray as shown. Note that the periodic structure pitch andillumination angle can be designed or adjusted so that the first orderrays entering the objective lens are closely aligned with the centraloptical axis. The rays illustrated in FIGS. 3(a) and 3(b) are shownsomewhat off axis, purely to enable them to be more easily distinguishedin the diagram.

At least the 0 and +1 orders diffracted by the target on substrate W arecollected by objective lens 16 and directed back through prism 15.Returning to FIG. 3(a), both the first and second illumination modes areillustrated, by designating diametrically opposite apertures labeled asnorth (N) and south (S). When the incident ray I is from the north sideof the optical axis, that is when the first illumination mode is appliedusing aperture plate 13N, the +1 diffracted rays, which are labeled+1(N), enter the objective lens 16. In contrast, when the secondillumination mode is applied using aperture plate 13S the −1 diffractedrays (labeled −1(S)) are the ones which enter the lens 16. Thus, in anembodiment, measurement results are obtained by measuring the targettwice under certain conditions, e.g., after rotating the target orchanging the illumination mode or changing the imaging mode to obtainseparately the −1^(st) and the +1^(st) diffraction order intensities.Comparing these intensities for a given target provides a measurement ofasymmetry in the target, and asymmetry in the target can be used as anindicator of a parameter of a lithography process, e.g., overlay error.In the situation described above, the illumination mode is changed.

A beam splitter 17 divides the diffracted beams into two measurementbranches. In a first measurement branch, optical system 18 forms adiffraction spectrum (pupil plane image) of the target on first sensor19 (e.g. a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements of thefirst order beam. The pupil plane image can also be used for manymeasurement purposes such as reconstruction, which are not described indetail here.

In the second measurement branch, optical system 20, 22 forms an imageof the target on the substrate W on sensor 23 (e.g. a CCD or CMOSsensor). In the second measurement branch, an aperture stop 21 isprovided in a plane that is conjugate to the pupil-plane. Aperture stop21 functions to block the zeroth order diffracted beam so that the imageDF of the target formed on sensor 23 is formed from the −1 or +1 firstorder beam. The images captured by sensors 19 and 23 are output to imageprocessor and controller PU, the function of which will depend on theparticular type of measurements being performed. Note that the term‘image’ is used here in a broad sense. An image of the periodicstructure features (e.g., grating lines) as such will not be formed, ifonly one of the −1 and +1 orders is present.

The particular forms of aperture plate 13 and stop 21 shown in FIG. 3are purely examples. In another embodiment of the invention, on-axisillumination of the targets is used and an aperture stop with anoff-axis aperture is used to pass substantially only one first order ofdiffracted radiation to the sensor. In yet other embodiments, 2nd, 3rdand higher order beams (not shown in FIG. 3) can be used inmeasurements, instead of or in addition to the first order beams.

In order to make the illumination adaptable to these different types ofmeasurement, the aperture plate 13 may comprise a number of aperturepatterns formed around a disc, which rotates to bring a desired patterninto place. Note that aperture plate 13N or 13S are used to measure aperiodic structure of a target oriented in one direction (X or Ydepending on the set-up). For measurement of an orthogonal periodicstructure, rotation of the target through 90° and 270° might beimplemented. Different aperture plates are shown in FIGS. 3(c) and (d).FIG. 3(c) illustrates two further types of off-axis illumination mode.In a first illumination mode of FIG. 3(c), aperture plate 13E providesoff-axis illumination from a direction designated, for the sake ofdescription only, as ‘east’ relative to the ‘north’ previouslydescribed. In a second illumination mode of FIG. 3(c), aperture plate13W is used to provide similar illumination, but from an oppositedirection, labeled ‘west’. FIG. 3(d) illustrates two further types ofoff-axis illumination mode. In a first illumination mode of FIG. 3(d),aperture plate 13NW provides off-axis illumination from the directionsdesignated ‘north’ and ‘west’ as previously described. In a secondillumination mode, aperture plate 13SE is used to provide similarillumination, but from an opposite direction, labeled ‘south’ and ‘east’as previously described. The use of these, and numerous other variationsand applications of the apparatus are described in, for example, theprior published patent application publications mentioned above.

FIG. 4 depicts an example composite metrology target formed on asubstrate. The composite target comprises four periodic structures (inthis case, gratings) 32, 33, 34, 35 positioned closely together. In anembodiment, the periodic structures are positioned closely togetherenough so that they all are within a measurement spot 31 formed by theillumination beam of the metrology apparatus. In that case, the fourperiodic structures thus are all simultaneously illuminated andsimultaneously imaged on sensors 19 and 23. In an example dedicated tooverlay measurement, periodic structures 32, 33, 34, 35 are themselvescomposite periodic structures (e.g., composite gratings) formed byoverlying periodic structures, i.e., periodic structures are patternedin different layers of the device formed on substrate W and such that atleast one periodic structure in one layer overlays at least one periodicstructure in a different layer. Such a target may have outer dimensionswithin 20 μm×20 μm or within 16 μm×16 μm. Further, all the periodicstructures are used to measure overlay between a particular pair oflayers. To facilitate a target being able to measure more than a singlepair of layers, periodic structures 32, 33, 34, 35 may have differentlybiased overlay offsets in order to facilitate measurement of overlaybetween different layers in which the different parts of the compositeperiodic structures are formed. Thus, all the periodic structures forthe target on the substrate would be used to measure one pair of layersand all the periodic structures for another same target on the substratewould be used to measure another pair of layers, wherein the differentbias facilitates distinguishing between the layer pairs. The meaning ofoverlay bias will be explained below, particularly with reference toFIG. 7.

FIGS. 7(a)-(c) show schematic cross sections of overlay periodicstructures (in this case gratings) of respective targets T, withdifferent biases. These can be used on substrate W, as seen in FIGS. 3and 4. Periodic structures with periodicity in the X direction are shownfor the sake of example only. Different combinations of these periodicstructures with different biases and with different orientations can beprovided.

Starting with FIG. 7(a), a composite overlay target 600 formed in twolayers, labeled L1 and L2, is depicted. In the bottom layer L1, a firstperiodic structure (in this case a grating) is formed by features (e.g.,lines) 602 and spaces 604 on a substrate 606. In layer L2, a secondperiodic structure (in this case a grating) is formed by features (e.g.,lines) 608 and spaces 610. (The cross-section is drawn such that thefeatures 602, 608 extend into the page.) The periodic structure patternrepeats with a pitch P in both layers. Lines 602 and 608 are mentionedfor the sake of example only, other types of features such as dots,blocks and via holes can be used. In the situation shown at FIG. 7(a),there is no overlay error and no bias, so that each feature 608 liesexactly over a feature 602 in the bottom periodic structure (where themeasurement is “line-on-line”—in an embodiment, no overlay error mayoccur where each feature 608 lies exactly over a space 610 wherein themeasurement is “line-on-trench”).

At FIG. 7(b), the same target with a bias +d is depicted such that thefeatures 608 of the upper periodic structure are shifted by a distance dto the right (the distance d being less than the pitch P), relative tothe features 602 of the lower periodic structures. That is, features 608and features 602 are arranged so that if they were both printed exactlyat their nominal locations, features 608 would be offset relative to thefeatures 602 by the distance d. The bias distance d might be a fewnanometers in practice, for example 10 nm 20 nm, while the pitch P isfor example in the range 300-1000 nm, for example 500 nm or 600 nm. AtFIG. 7(c), the same target with a bias −d is depicted such that thefeatures 608 are shifted to the left relative to the features 602.Biased targets of this type shown at FIGS. 7(a) to (c), and their use inmeasurement, are described in, for example, the patent applicationpublications mentioned above.

Further, as alluded to above, while FIGS. 7(a)-(c) depicts the features608 lying over the features 602 (with or without a small bias of +d or−d applied), which is referred to as a “line on line” target having abias in the region of zero, a target may have a programmed bias of P/2,that is half the pitch, such that each feature 608 in the upper periodicstructure lies over a space 604 in the lower periodic structure. This isreferred to as a “line on trench” target. In this case, a small bias of+d or −d may also be applied. The choice between “line on line” targetor a “line on trench” target depends on the application.

Returning to FIG. 4, periodic structures 32, 33, 34, 35 may also differin their orientation, as shown, so as to diffract incoming radiation inX and Y directions. In one example, periodic structures 32 and 34 areX-direction periodic structures with biases of +d, −d, respectively.Periodic structures 33 and 35 may be Y-direction periodic structureswith offsets +d and −d respectively. While four periodic structures areillustrated, another embodiment may include a larger matrix to obtaindesired accuracy. For example, a 3×3 array of nine composite periodicstructures may have biases −4d, −3d, −2d, −d, 0, +d, +2d, +3d, +4d.Separate images of these periodic structures can be identified in theimage captured by sensor 23.

FIG. 5 shows an example of an image that may be formed on and detectedby the sensor 23, using the target of FIG. 4 in the apparatus of FIG. 3,using the aperture plates 13NW or 13SE from FIG. 3(d). While the sensor19 cannot resolve the different individual periodic structures 32 to 35,the sensor 23 can do so. The dark rectangle represents the field of theimage on the sensor, within which the illuminated spot 31 on thesubstrate is imaged into a corresponding circular area 41. Within this,rectangular areas 42-45 represent the images of the periodic structures32 to 35. If the periodic structures are located in product areas,product features may also be visible in the periphery of this imagefield. Image processor and controller PU processes these images usingpattern recognition to identify the separate images 42 to 45 of periodicstructures 32 to 35. In this way, the images do not have to be alignedvery precisely at a specific location within the sensor frame, whichgreatly improves throughput of the measuring apparatus as a whole.

Once the separate images of the periodic structures have beenidentified, the intensities of those individual images can be measured,e.g., by averaging or summing selected pixel intensity values within theidentified areas. Intensities and/or other properties of the images canbe compared with one another. These results can be combined to measuredifferent parameters of the lithographic process. Overlay performance isan example of such a parameter.

FIG. 6 illustrates how, using for example the method described in PCTpatent application publication no. WO 2011/012624, overlay error betweenthe two layers containing the component periodic structures 32 to 35 ismeasured through asymmetry of the periodic structures, as revealed bycomparing their intensities in the +1 order and −1 order dark fieldimages. At step M1, the substrate, for example a semiconductor wafer, isprocessed through the lithographic cell of FIG. 2 one or more times, tocreate a structure including the target comprising periodic structures32-35. At M2, using the metrology apparatus of FIG. 3, an image of theperiodic structures 32 to 35 is obtained using one of the first orderdiffracted beams (say −1). In an embodiment, a first illumination mode(e.g., the illumination mode created using aperture plate 13NW) is used.Then, whether by, for example, changing the illumination mode, orchanging the imaging mode, or by rotating substrate W by 180° in thefield of view of the metrology apparatus, a second image of the periodicstructures using the other first order diffracted beam (+1) can beobtained (step M3). Consequently, the +1 diffracted radiation iscaptured in the second image. In an embodiment, the illuminated mode ischanged and a second illumination mode (e.g., the illumination modecreated using aperture plate 13SE) is used. In an embodiment,tool-induced artifacts like TIS (Tool Induced Shift) can be removed bydoing the measurement at 0° and 180° substrate orientation.

Note that, by including only half of the first order diffractedradiation in each image, the ‘images’ referred to here are notconventional dark field microscopy images. The individual periodicstructure features are not resolved. Each periodic structure will berepresented simply by an area of a certain intensity level. In step M4,a region of interest (ROI) is identified within the image of eachcomponent periodic structure, from which intensity levels will bemeasured.

Having identified the region of interest P1, P2, P3, P4 for eachrespective individual periodic structure 32-35 and measured itsintensity, the asymmetry of the periodic structure, and hence, e.g.,overlay error, can then be determined. This is done by the imageprocessor and controller PU in step M5 comparing the intensity valuesobtained for +1 and −1 orders for each periodic structure 32-35 toidentify any difference in their intensity, i.e., an asymmetry. The term“difference” is not intended to refer only to subtraction. Differencesmay be calculated in ratio form. In step M6 the measured asymmetries fora number of periodic structures are used together with, if applicable,knowledge of the overlay biases of those periodic structures tocalculate one or more performance parameters of the lithographic processin the vicinity of the target T. A performance parameter of interest isoverlay. Other parameters of performance of the lithographic process canbe calculated such as focus and/or dose. The one or more performanceparameters can be fed back for improvement of the lithographic process,used to improve the measurement and calculation process of FIG. 6itself, used to improve the design of the target T, etc.

In an embodiment to determine overlay, FIG. 8 depicts a curve 702 thatillustrates the relationship between overlay error OV and measuredasymmetry A for an ‘ideal’ target having zero offset and no structuralasymmetry within the individual periodic structures forming the overlaytarget. These graphs are to illustrate the principles of determining theoverlay only, and in each graph, the units of measured asymmetry A andoverlay error OV are arbitrary.

In the ‘ideal’ situation of FIGS. 7(a)-(c), the curve 702 indicates thatthe measured asymmetry A has a sinusoidal relationship with the overlay.The period P of the sinusoidal variation corresponds to the period(pitch) of the periodic structures, converted of course to anappropriate scale. The sinusoidal form is pure in this example, but caninclude harmonics in real circumstances. For the sake of simplicity, itis assumed in this example (a) that only first order diffractedradiation from the target reaches the image sensor 23 (or its equivalentin a given embodiment), and (b) that the experimental target design issuch that within these first orders a pure sine-relation exists betweenintensity and overlay between upper and lower periodic structuresresults. Whether this is true in practice is a function of the opticalsystem design, the wavelength of the illuminating radiation and thepitch P of the periodic structure, and the design and stack of thetarget.

As mentioned above, biased periodic structures can be used to measureoverlay, rather than relying on a single measurement. This bias has aknown value defined in the patterning device (e.g. a reticle) from whichit was made, that serves as an on-substrate calibration of the overlaycorresponding to the measured signal. In the drawing, the calculation isillustrated graphically. In steps M1-M5 of FIG. 6, asymmetrymeasurements A_(+d) and A_(−d) are obtained for component periodicstructures having biases +d an −d respectively (as shown in FIGS. 7(b)and 7(c), for example). Fitting these measurements to the sinusoidalcurve gives points 704 and 706 as shown. Knowing the biases, the trueoverlay error OV can be calculated. The pitch P of the sinusoidal curveis known from the design of the target. The vertical scale of the curve702 is not known to start with, but is an unknown factor which we cancall an overlay proportionality constant, K.

In equation terms, the relationship between overlay error OV_(E) andintensity asymmetry A is assumed to be:A _(±d) =K sin(OV_(E) ±d)where overlay error OV_(E) is expressed on a scale such that the targetpitch P corresponds to an angle 2π radians. The term d is the gratingbias of the target (or sub-target) being measured. Using twomeasurements of targets with different, known biases (e.g. +d and −d),the overlay error OV_(E) can be calculated using:

${O\; V_{E}} = {{atan}\left( {\frac{A_{+ d} + A_{- d}}{A_{+ d} - A_{- d}} \cdot {\tan(d)}} \right)}$where A_(+d) is an intensity asymmetry measurement of the +d biasedtarget and A_(−d) is an intensity asymmetry measurement of the −d biasedtarget.

Although these measurement techniques are fast and relativelycomputationally simple (once calibrated), they rely on an assumptionthat the overlay/lateral shift is the only cause of asymmetry. That is,it assumes an ‘ideal’ situation with, for example, no structuralasymmetry in the target. Any structural asymmetry in the stack, such asasymmetry of features within one or both of the overlaid periodicstructures, also causes an asymmetry in the 1^(st) orders besides theoverlay/lateral shift. This structural asymmetry which is not related tothe overlay clearly perturbs the measurement, giving an inaccurateresult.

As an example of structural asymmetry, one or more of the periodicstructures of the target may be structurally deformed. For example, oneor more side walls of periodic structure features (e.g., grating lines)of the target may not be vertical as intended. As another example, oneor spaces between periodic structure features (e.g., grating spaces oftrenches) of a target may be larger or smaller than as intended.Further, one or more features of a periodic structure of a target (e.g.,grating lines) may have a smaller or larger width than as intended.Additionally, even where a difference from intended is uniform for oneor more periodic structures of the target, that difference from intendedmay not be the same as for one or more other periodic structures of thetarget. Structural asymmetry in the lower periodic structure of acomposite target is a common form of structural asymmetry. It mayoriginate, for example, in the substrate processing steps such aschemical-mechanical polishing (CMP), performed after the lower periodicstructure was originally formed.

Referring to FIG. 7(d), an example of structural asymmetry of a lowerperiodic structure is schematically depicted. The features and spaces inthe periodic structures at FIG. 7(a) to (c) are shown as perfectlysquare-sided, when a real feature and space would have some slope on asurface, and a certain roughness. Nevertheless they are intended to beat least symmetrical in profile. The features 602 and/or spaces 604 atFIG. 7(d) in the lower periodic structure no longer have a symmetricalform at all, but rather have become distorted by, for example, one ormore processing steps. Thus, for example, a bottom surface of each space604 has become tilted. Side wall angles of the features and spaces havebecome asymmetrical also. When overlay is measured by the method of FIG.6 using only two biased periodic structures, the structural asymmetrycannot be distinguished from overlay, and overlay measurements becomeunreliable as a result.

It has been further discovered that, in addition to or alternatively tostructural asymmetry in a target, a stack difference between adjacentperiodic structures of a target or between adjacent targets may be afactor that adversely affects the accuracy of measurement, such asoverlay measurement. Stack difference may be understood as anun-designed difference in physical configurations between adjacentperiodic structures or targets. Stack difference causes a difference inan optical property (e.g., intensity, polarization, etc.) of measurementradiation between the adjacent periodic structures or targets that isdue to a cause other than overlay error, other than intentional bias andother than structural asymmetry common to the adjacent periodicstructures or targets. Stack difference includes, but is not limited to,a thickness difference between the adjacent periodic structures ortargets (e.g., a difference in thickness of one or more layers such thatone periodic structure or target is higher or lower than anotherperiodic structure or target designed to be at a substantially equallevel), a refractive index difference between the adjacent periodicstructures or targets (e.g., a difference in refractive index of one ormore layers such that the combined refractive index for the one or morelayers for one periodic structure or target is different than thecombined refractive index for the one or more layers for of anotherperiodic structure or target even though designed to have asubstantially equal combined refractive index), a difference in materialbetween the adjacent periodic structures or targets (e.g., a differencein the material type, material uniformity, etc. of one or more layerssuch that there is a difference in material for one periodic structureor target from another periodic structure or target designed to have asubstantially same material), a difference in the grating period of thestructures of adjacent periodic structures or targets (e.g., adifference in the grating period for one periodic structure or targetfrom another periodic structure or target designed to have asubstantially same grating period), a difference in depth of thestructures of adjacent periodic structures or targets (e.g., adifference due to etching in the depth of structures of one periodicstructure or target from another periodic structure or target designedto have a substantially same depth), a difference in width (CD) of thefeatures of adjacent periodic structures or targets (e.g., a differencein the width of features of one periodic structure or target fromanother periodic structure or target designed to have a substantiallysame width of features), etc. In some examples, the stack difference isintroduced by processing steps, such as CMP, layer deposition, etching,etc. in the patterning process. In an embodiment, periodic structures ortargets are adjacent if within 200 μm of each other, within 150 μm ofeach other, within 100 μm of each other, within 75 μm of each other,within 50 μm of each other, within 40 μm of each other, within 30 μm ofeach other, within 20 μm of each other, or within 10 μm of each other.

The effect of stack difference (which can be referred to as gratingimbalance between gratings) on intensity asymmetry measurements A_(+d),A_(−d) (where the subscript indicates the target bias of the targetareas corresponding to the ROIs) can be generally formulated as:A _(+d)=(K+ΔK)sin(OV_(E) +d)A _(−d)=(K−ΔK)sin(OV_(E) −d)wherein ΔK represents a difference in the overlay sensitivityattributable to the stack difference. And so, the overlay error OV_(E)(assuming it is small) can be proportional to

$\frac{\Delta\; K}{K}{d.}$

Stack difference may be considered to be a spatial stack parametervariation, i.e., a stack parameter variation over the substrate(target-to-target). Another issue which may be encountered is stackparameter process drift, where one or more of the stack parameters of atarget drift from optimal over time, due to process drift. This can beconsidered to be a temporal stack parameter variation.

Now, in the face of structural asymmetry, stack difference, stackparameter process drift and any other process variabilities, it isdesirable to derive a combination of target layout, measurement beamwavelength, measurement beam polarization, etc. that would yield anaccurate measurement of the desired process parameter (e.g., overlay)and/or that yields measurement values of the desired process parameterthat is robust to process variability. Thus, it is desirable, forexample, to perform measurements using a desirably optimum selection ofa target-measurement parameter combination so as to obtain more accurateprocess parameter measurement and/or that yields measurement values ofthe desired process parameter that is robust to process variability.This is because the measurement accuracy and/or sensitivity of thetarget may vary with respect to one or more attributes of the targetitself and/or one or more attributes of the measurement radiationprovided onto the target; for example: the wavelength of the radiation,the polarization of the radiation, and/or the intensity distribution(i.e., angular or spatial intensity distribution) of the radiation. Inan embodiment, the wavelength range of the radiation is limited to oneor more wavelengths selected from a range (e.g., selected from the rangeof about 400 nm to 900 nm). Further, a selection of differentpolarizations of the radiation beam may be provided and variousillumination shapes can be provided using, for example, a plurality ofdifferent apertures. As such, it is desirable to determine a measurementprofile which is optimized for a particular target.

The measurement profile comprises one or more parameters of themeasurement itself, the one or more parameters of the measurement itselfcan include one or more parameters relating to a measurement beam and/ormeasurement apparatus used to make the measurement. For example, if themeasurement used in a substrate measurement recipe is adiffraction-based optical measurement, one or more parameters of themeasurement itself may include a wavelength of measurement radiation,and/or a polarization of measurement radiation, and/or measurementradiation intensity distribution, and/or an illumination angle (e.g.,incident angle, azimuth angle, etc.) relative to the substrate ofmeasurement radiation, and/or the relative orientation relative to apattern on the substrate of diffracted measurement radiation, and/ornumber of measured points or instances of the target, and/or thelocations of instances of the target measured on the substrate. The oneor more parameters of the measurement itself may include one or moreparameters of the metrology apparatus used in the measurement, which caninclude detector sensitivity, numerical aperture, etc.

In this context, a pattern measured (also referred to as a “target” or“target structure”) may be a pattern that is optically measured, e.g.,whose diffraction is measured. The pattern measured may be a patternspecially designed or selected for measurement purposes. Multiple copiesof a target may be placed on many places on a substrate. For example, asubstrate measurement recipe may be used to measure overlay. In anembodiment, a substrate measurement recipe may be used to measureanother process parameter (e.g., dose, focus, CD, etc.) In anembodiment, a measurement profile may be used for measuring alignment ofa layer of a pattern being imaged against an existing pattern on asubstrate; for example, a measurement profile may be used to align thepatterning device to the substrate, by measuring a relative position ofthe substrate.

A number of methods have been described for evaluating and optimizingtarget-measurement parameter combinations. Such methods are performed inadvance of production. Therefore, once optimized, the chosentarget-measurement parameter combination(s) will typically be usedthroughout a production run, i.e., a predetermined measurement profilewill be used to measure a target of a corresponding target design inaccordance with a predetermined target-measurement parametercombination. However, as discussed, there may be un-designed stackparameter variation in the target, leading to stack difference betweentargets and/or stack parameter process drift. For example, layerthickness of one or more layers within the stack may vary over thesubstrate (i.e., target-to-target) and/or over time (i.e., drift). Oneconsequence of this stack parameter variation may be that themeasurement profile is no longer optimal for the target. This can resultin measurements of the target being inaccurate. Stack parametervariation may also be an indication of process control issues (e.g.,process drift) generally, and therefore may be a useful processmonitoring metric in itself.

Methods for evaluating and optimizing target-measurement parametercombinations may comprise those which analyze target response sequencedata describing the variation of target response with variation in themeasurement profile, in particular one or more parameters of themeasurement radiation such as wavelength (e.g., spectral sequence data).In an embodiment, the target response sequence data can represent anoscillatory dependence of measured data (e.g., an intensity metricobtained as field data (at an image plane) or pupil data (at pupilplane)) as a function of measurement radiation wavelength. FIG. 9 is anexample graph of data for a target for measurement of an intensitymetric, in this specific example overlay sensitivity K, at variouswavelengths λ, for a single polarization (in this case, linear Xpolarization). A curve K(λ) has been fitted through the data and so thisrepresentation can be called a swing curve. As will be appreciated, agraph need not be generated as just the data can be processed. A similargraph of data can be constructed for the same target for measurement atthe various wavelengths for a different single polarization (e.g.,linear Y polarization). In FIG. 9, stack sensitivity and overlaysensitivity are graphed for various measurement beam wavelengths.Further, while the polarizations here is linear X polarization, it canbe a different polarization (such as linear Y polarization, left-handedelliptically polarized radiation, right-handed elliptically polarizedradiation, etc.)

The intensity metric may be any suitable metric derived from thedetected intensities, e.g., intensity asymmetry, overlay sensitivity Kor stack sensitivity (SS) (also signal contrast). Stack sensitivity canbe understood as a measure of how much the intensity of the signalchanges as overlay changes because of diffraction between target (e.g.,grating) layers. That is, in an overlay context, it detects the contrastbetween upper and lower periodic structure of an overlay target and thusrepresents a balance between diffraction efficiencies between the upperand lower periodic structure. It is thus an example measure ofsensitivity of the measurement. In an embodiment, stack sensitivity isthe ratio between intensity asymmetry and average intensity. In anembodiment, stack sensitivity can be formulated as SS=KL/I_(M), whereinL is a user defined constant (e.g., in an embodiment, the value L is 20nm and/or the value of the bias d) and I_(M) is the mean intensity ofthe measurement beam diffracted by the target.

The example of FIG. 9 shows a swing curve for overlay sensitivity K(λ)as a function of wavelength λ, where

${K(\lambda)} = \frac{{A(\lambda)}_{+ d} - {A(\lambda)}_{- d}}{2d*{{df}(\lambda)}}$A(λ)_(+d) and A(λ)_(−d) are the intensity asymmetry measurementscorresponding to biases +d and −d respectively, as a function ofwavelength and df(λ) is a dose factor as a function of wavelength. Thedose factor may be any function of source intensity and measurementtime. In a specific embodiment, it may comprise the product of sourceintensity and integration time as a function of wavelength.

It is proposed to use such swing curves to monitor measurement validityduring production. Monitoring measurement validity may comprisedetermining whether a measurement profile remains optimal (e.g., withina threshold margin) for a target being measured. This method maycomprise comparing measured target response sequence data (e.g., ameasured swing curve) obtained during production to previously storedreference target response sequence data (e.g., a reference swing curve).The reference target response sequence data may have been obtained in aprevious optimization stage, when determining an optimaltarget-measurement parameter combination, and therefore may representthe target response sequence data which would be obtained if measuringthe target as designed (i.e., with little or no un-designed variation orerror in stack parameters). If the comparison shows that the measuredtarget response sequence data differs too greatly from the referencetarget response sequence data, then it may be decided that measurementsof that target using a corresponding measurement profile in accordancewith a previously determined optimized target-measurement parametercombination will be unreliable and a corrective action instigated.

It is further proposed, in an embodiment, that the swing curves can alsobe used to monitor stack parameters (e.g., one or more layer heights(i.e., layer thicknesses), refractive indices and/or absorptances)during production. This can provide additional process monitoring dataessentially “for free”, from (e.g., μDBO) measurements performed in anycase to monitor overlay. The ability to measure such stack parameterscan improve process control during fabrication. Presently suchmeasurements are optically estimated using multi thin-film targets (withno gratings present). However, this requires valuable additional area onthe substrate, and additional time to perform the measurements.

FIG. 10 is a flowchart of a method according to an embodiment. At step1000, one or more (e.g., similar) targets at one or more differentlocations on a substrate are measured using a plurality of differentmeasurement profiles (e.g., over a plurality of different measurementradiation wavelengths). The targets may comprise μDBO targets.Preferably, measurement data over the different measurement profiles isobtained simultaneously for each target. For example, each measurementsmay be performed using broadband measurement radiation covering thedifferent measurement profiles. The broadband radiation may comprise acontinuous spectrum or a plurality of different discrete wavelengths(and/or polarizations).

At step 1010, measurement data obtained at step 1000 is used todetermine measured target response sequence data (e.g., a measured swingcurve comprising a spectral sequence).

At step 1020, the measured target response sequence data is compared toreference target response sequence data 1030 (e.g., a reference swingcurve comprising a spectral sequence). The reference target responsesequence data may have been obtained in a previous optimization step orfrom a previous measurement and then stored, and may comprise targetresponse sequence data for the target as designed. The target responsesequence data varies with variation in stack parameters (e.g., layerthicknesses in the stack) and therefore this comparison is indicative ofthe degree of difference between the actual measured target and thedesigned target.

In an embodiment, step 1020 may comprise using a suitable comparisonalgorithm to perform the comparison. An example of a suitable comparisonalgorithm is a dynamic time warping (DTW) algorithm, although otheralgorithms may also be used. The result of the comparison may be asimilarity metric describing the degree of similarity between themeasured target response sequence data and reference target responsesequence data.

At step 1040, a validity check is performed to determine whethermeasurements of the target performed in accordance with a previouslydetermined optimized target-measurement parameter combination will bevalid. This may comprise comparing the similarity metric determined atstep 1030 to a threshold similarity metric. If this comparison indicatesthat the sequences compared at step 1020 are too dissimilar, thevalidity check is negative (i.e., measurements will be invalid asunreliable), otherwise the validity check will be positive andmeasurements of the target may be continued with the previouslydetermined optimized target-measurement parameter combination. Aninvalid validity check indicates that one or more of the stackparameters have changed significantly from that designed; if so, anypreviously performed optimization of the measurement profile based onthe target as designed may be deemed invalid. The user may then beinformed of this and prompted to perform a further optimization and/ortarget selection or other corrective action. In an embodiment, thefurther optimization may be performed in-line (assuming there aresufficient samples), so as to determine an updated optimal measurementprofile “on the spot” based on the swing curve peaks. An invalidvalidity check may be an indicator of process drift in the lithographyprocess in forming the target (and therefore in forming the productstructures) and therefore may be used as a prompt for corrective actionin the lithographic process.

An optional stage 1050 comprises determining values for variations inone or more of the stack parameters from the target response sequencedata. This comprises using a suitable model for stack responsecalculation (if available) and initial stack parameter data to calculatea model which best fits the observed swing curve offset. This stage maycomprise, at step 1060, constructing an error function from the measuredtarget response sequence data and reference target response sequencedata using the comparison algorithm (e.g., DTW). At step 1070, aninitial stack estimate 1075 and suitable stack response model are usedto simulate a swing curve for the model stack (simulated target responsesequence data). The initial stack estimate 1075 may be based on thetarget stack as designed, i.e., the relevant stack parameters may bethose as designed.

At step 1080, the error function from step 1060 is minimized usingstarting conditions defined at step 1070, to find the stack parameterswhich fit the swing curve offset. This step may comprise using asuitable optimization algorithm. An example of a suitable optimizationalgorithm is a Nelder-Mead Simplex algorithm. The error being minimizedmay comprise the difference between the measured swing curve and thesimulated swing curve (i.e., the simulated target response sequence datafrom the model which is being minimized). Alternatively, the error beingminimized may comprise the difference between the measured swing curveoffset (difference between measured swing curve and reference swingcurve) and a simulated swing curve offset (difference between simulatedswing curve and reference swing curve). In a specific example, theminimization may comprise minimizing an L2 norm (or other suitable norm)of the comparison function (warping function) found by the comparison(e.g., DTW) algorithm.

In summary the concepts described herein enable inline monitoring ofmeasurement validity, enabling fast flagging of suboptimal (e.g., μDBO)measurement conditions. This can be particularly useful in R&Denvironments where there are many process changes. The concepts alsoenable inline monitoring of stack parameter variation from (alreadyroutinely performed) overlay measurements, thereby reducing totalmetrology time. In addition, in-line profile optimization based on theswing curve peaks is possible (accepting the limited spatial sampling).If there is a significant jump or change in the swing curve shape, a newoptimal measurement profile can be generated on the spot (assuming thereare sufficient samples). In principle it is not necessary to stopproduction and execute a full holistic metrology qualification run toselect new profile.

Those measurements made using the target naturally may be used increating, for example, devices by a lithographic process. Further,besides being used to correct measurements made using the target, themeasure of the asymmetric deformation of the target may be used in the(re-)design of the target (e.g., making a change to a layout of thedesign), may be used in the process of forming the target (e.g., makinga change in material, a change in printing steps or conditions, etc.),may be used in formulation of the measurement conditions (e.g., make achange in the optical measurement formulation in terms of wavelength,polarization, illumination mode, etc. of the measurement beam), etc.

Some embodiments according to the invention are provided in belownumbered clauses:

-   1. A process monitoring method comprising:    -   obtaining measured target response sequence data relating to a        measurement response of one or more targets formed on a        substrate by a lithographic process to measurement radiation        comprising a plurality of measurement profiles;    -   obtaining reference target response sequence data relating to a        measurement response of said one or more targets as designed to        said measurement radiation;    -   comparing said measured target response sequence data and        reference target response sequence data; and    -   performing a process monitoring action based on the comparison        of said measured target response sequence data and reference        target response sequence data.-   2. A method according to clause 1, wherein said comparison step    comprises determining a similarity metric relating to the similarity    of the measured target response sequence data to the reference    target response sequence data.-   3. A method according to clause 2, wherein    -   said comparison step comprises comparing said similarity metric        to a threshold similarity metric; and    -   said step of performing a process monitoring action comprises        determining a measurement validity based on the comparing of the        similarity metric to the threshold similarity metric, the        measurement validity relating to a validity of measurements of        said one or more targets using measurement radiation having a        corresponding measurement profile in accordance with a        target-measurement parameter combination.-   4. A method according to clause 3, wherein said target-measurement    parameter combination comprises an optimized target-measurement    parameter combination determined in a previous optimization step.-   5. A method according to clause 4, further comprising performing    said optimization step.-   6. A method according to clause 4 or 5, wherein said reference    target response sequence data is determined in said optimization    step.-   7. A method according to any of clauses 3 to 6, wherein, where the    comparison of said similarity metric to the threshold similarity    metric indicates too great a dissimilarity, said measurement    validity is deemed to be invalid, otherwise said measurement    validity is deemed to be valid.-   8. A method according to clause 7, wherein, where said measurement    validity is deemed to be invalid, said step of performing a process    monitoring action comprises performing an optimization update step    to determine a measurement profile optimized to said one or more    targets as measured.-   9. A method according to clause 8, wherein, said optimization update    step comprises an in-line measurement profile optimization based    upon said measured target response sequence data.-   10. A method according to clause 7, 8 or 9, wherein, where said    measurement validity is deemed to be invalid, said step of    performing a process monitoring action comprises selecting a    different one or more targets for measurement.-   11. A method according to any of clauses 7 to 10, wherein, where    said measurement validity is deemed to be invalid, said step of    performing a process monitoring action comprises performing a    corrective action to said lithographic process.-   12. A method according to any preceding clause, wherein the step of    comparing the measured target response sequence data to the    reference target response sequence data is performed using a dynamic    time warping algorithm.-   13. A method according to any preceding clause, wherein said    measured target response sequence data and reference target response    sequence data each comprise data describing the variation of the    target response with measurement radiation wavelength.-   14. A method according to any preceding clause, wherein the measured    target response sequence data and reference target response sequence    data is determined in terms of an intensity metric.-   15. A method according to clause 14, wherein said intensity metric    comprises an intensity asymmetry metric, derived from the intensity    difference between corresponding pairs of non-zeroth diffraction    orders.-   16. A method according to clause 15, wherein said intensity    asymmetry metric comprises overlay sensitivity, said overlay    sensitivity comprising a proportionality constant in a relationship    between a function of an overlay offset between periodic structures    of a target and said intensity difference.-   17. A method according to clause 15, wherein said intensity    asymmetry metric comprises stack sensitivity, wherein stack    sensitivity is the ratio of the overlay sensitivity to a measured    average intensity, said overlay sensitivity comprising a    proportionality constant in a relationship between a function of an    overlay offset between periodic structures of a target and said    intensity difference.-   18. A method according to any preceding clause, further comprising    determining values for one or more stack parameters of said one or    more targets formed on the substrate from said measured target    response sequence data.-   19. A method according to clause 18, wherein said determining values    for one or more stack parameters comprises:    -   determining a suitable target model parameterized at least in        part by said stack parameters;    -   performing a simulated measurement of said target model to        obtain a simulated target response sequence data; and    -   minimizing the difference between said simulated target response        sequence data and said measured target response sequence data.-   20. A method according to clause 19, wherein said minimization step    comprises devising an error function for minimizing the difference    between said simulated target response sequence data and said    measured target response sequence data.-   21. A method according to any of clauses 18 to 20, wherein said one    or more stack parameters comprise at least one layer height of a    layer comprised in the target.-   22. A method according to any of clauses 18 to 20, wherein said one    or more stack parameters comprise a plurality of layer heights of    different layers comprised in the target.-   23. A method according to any preceding clause, wherein said    measured target response sequence data relates to one or more    dark-field measurements of said one or more targets wherein said at    least one corresponding pair of non-zeroth diffraction orders is    detected in an image plane.-   24. A method according to any preceding clause, wherein each of said    one or more targets each comprises at least two sub-targets, each    sub-target having a different imposed overlay bias.-   25. A method according to any preceding clause, comprising    performing a measurement of said one or more targets to obtain said    measured target response sequence data.-   26. A metrology apparatus comprising:    -   an illumination system configured to illuminate one or more        targets formed on a substrate by a lithographic process with        measurement radiation comprising a plurality of measurement        profiles;    -   a detection system configured to detect scattered radiation        arising from illumination of said one or more targets; and    -   a processor operable to:        -   derive measured target response sequence data from the            detected scattered radiation; and        -   compare said measured target response sequence data to            reference target response sequence data relating to a            measurement response of said one or more targets as designed            to said measurement radiation.-   27. A metrology apparatus according to clause 26, being operable to    perform a process monitoring action based on the comparison of said    measured target response sequence data and reference target response    sequence data.-   28. A metrology apparatus according to clause 27, wherein said    processor is operable to perform said comparison by determining a    similarity metric relating to the similarity of the measured target    response sequence data to the reference target response sequence    data.-   29. A metrology apparatus according to clause 28, wherein said    processor is operable to:    -   perform said comparison by comparing said similarity metric to a        threshold similarity metric; and    -   determine a measurement validity based on the comparison of the        similarity metric to the threshold similarity metric, the        measurement validity relating to a validity of measurements of        said one or more targets using measurement radiation having a        corresponding measurement profile in accordance with a        target-measurement parameter combination.-   30. A metrology apparatus according to clause 29, wherein said    target-measurement parameter combination comprises a predetermined    optimized target-measurement parameter combination.-   31. A metrology apparatus according to clause 30, being operable to    optimize the target-measurement parameter combination.-   32. A metrology apparatus according to clause 29, 30 or 31, wherein    said processor is operable, where the comparison of said similarity    metric to the threshold similarity metric indicates too great a    dissimilarity, to deem said measurement validity to be invalid,    otherwise to deem said measurement validity to be valid.-   33. A metrology apparatus according to clause 32, wherein, where    said measurement validity is deemed to be invalid, said processor is    operable to perform an optimization update to determine a    measurement profile optimized to said one or more targets as    measured.-   34. A metrology apparatus according to clause 33, wherein, said    optimization update comprises an in-line measurement profile    optimization based upon said measured target response sequence data.-   35. A metrology apparatus according to clause 32, 33 or 34, wherein,    where said measurement validity is deemed to be invalid, said    processor is operable to select a different one or more targets for    measurement.-   36. A metrology apparatus according to any of clauses 32 to 35,    wherein, where said measurement validity is deemed to be invalid,    said processor is operable to determine a correction for said    lithographic process.-   37. A metrology apparatus according to any of clauses 26 to 36,    wherein the step of comparing the measured target response sequence    data to the reference target response sequence data is performed    using a dynamic time warping algorithm.-   38. A metrology apparatus according to any of clauses 26 to 37,    wherein said measured target response sequence data and reference    target response sequence data each comprise data describing the    variation of the target response with measurement radiation    wavelength.-   39. A metrology apparatus according to any of clauses 26 to 38,    wherein the processor is operable to determine the measured target    response sequence data and reference target response sequence data    in terms of an intensity metric.-   40. A metrology apparatus according to clause 39, wherein said    intensity metric comprises an intensity asymmetry metric, and the    processor is operable to derive the intensity asymmetry metric from    the intensity difference between corresponding pairs of non-zeroth    diffraction orders.-   41. A metrology apparatus according to clause 40, wherein said    intensity asymmetry metric comprises overlay sensitivity, said    overlay sensitivity comprising a proportionality constant in a    relationship between a function of an overlay offset between    periodic structures of a target and said intensity difference.-   42. A metrology apparatus according to clause 40, wherein said    intensity asymmetry metric comprises stack sensitivity, wherein    stack sensitivity is the ratio of the overlay sensitivity to a    measured average intensity, said overlay sensitivity comprising a    proportionality constant in a relationship between a function of an    overlay offset between periodic structures of a target and said    intensity difference.-   43. A metrology apparatus according to any of clauses 26 to 42,    wherein the processor is operable to determine values for one or    more stack parameters of said one or more targets formed on the    substrate from said measured target response sequence data.-   44. A metrology apparatus according to clause 43, wherein the    processor is operable to:    -   determine a suitable target model parameterized at least in part        by said stack parameters;    -   perform a simulated measurement of said target model to obtain a        simulated target response sequence data; and    -   minimize the difference between said simulated target response        sequence data and said measured target response sequence data to        determine said one or more stack parameters.-   45. A metrology apparatus according to clause 44, wherein the    processor is operable to devise an error function for minimizing the    difference between said simulated target response sequence data and    said measured target response sequence data.-   46. A metrology apparatus according to any of clauses 43 to 45,    wherein said one or more stack parameters comprise at least one    layer height of a layer comprised in the target.-   47. A metrology apparatus according to any of clauses 43 to 45,    wherein said one or more stack parameters comprise a plurality of    layer heights of different layers comprised in the target.-   48. A metrology apparatus according to any of clauses 26 to 47,    being operable to perform one or more dark-field measurements of    said one or more targets wherein said scattered radiation comprises    at least one corresponding pair of non-zeroth diffraction orders    which is detected by the detection system in an image plane.-   49. A metrology apparatus according to any of clauses 26 to 48,    wherein each of said one or more targets each comprises at least two    sub-targets, each sub-target having a different imposed overlay    bias.-   50. A computer program comprising program instructions operable to    perform the method according to any of clauses 1 to 25 when run on a    suitable apparatus.-   51. A non-transient computer program carrier comprising the computer    program of clause 50.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments reveals thegeneral nature of embodiments of the invention such that others can, byapplying knowledge within the skill of the art, readily modify and/oradapt for various applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The invention claimed is:
 1. A method of metrology in a metrologyapparatus for monitoring a measurement process, comprising: obtainingmeasured target response sequence data relating to a measurementresponse of one or more targets formed on a substrate by a lithographicprocess to measurement radiation comprising a plurality of measurementprofiles, wherein the measured target response sequence data describes avariation of the measurement response of the one or more targets inresponse to variations of the plurality of measurement profiles;obtaining reference target response sequence data relating to ameasurement response of the one or more targets as designed to themeasurement radiation, wherein the reference target response sequencedata describes an optimal measurement response of the one or moretargets in response to a designed plurality of measurement profileswithout un-designed variation; comparing the measured target responsesequence data and the reference target response sequence data; anddetermining values for variations in one or more stack parameters of theone or more targets from the measured target response sequence databased on the comparison.
 2. The method of claim 1, wherein determiningthe values for variations in the one or more stack parameters comprises:constructing an error function from the measured target responsesequence data and reference target response sequence data based on thecomparison; generating simulated target response sequence data;minimizing the error function using the simulated target responsesequence data as an initial condition to determine the values forvariations in the one or more stack parameters.
 3. The method of claim2, wherein generating the simulated target response sequence datacomprises: determining a target stack response model parameterized atleast in part by the stack parameters; and performing a simulatedmeasurement of the target stack response model to obtain the simulatedtarget response sequence data.
 4. The method of claim 2, whereinminimizing the error function comprises: minimizing a difference betweenthe simulated target response sequence data and the measured targetresponse sequence data.
 5. The method of claim 2, wherein minimizing theerror function comprises: calculating a first difference between themeasured target response sequence data and the reference target responsesequence data; calculating a second difference between the simulatedtarget response sequence data and the reference target response sequencedata; and minimizing a third difference between first difference and thesecond difference.
 6. The method of claim 2, wherein minimizing theerror function comprises using a Nelder-Mead Simplex algorithm tominimize the error function.
 7. The method of claim 2, wherein comparingthe measured target response sequence data and the reference targetresponse sequence data comprises: performing a dynamic time warpingalgorithm to determining the warping function to describe a similarityof the measured target response sequence data to the reference targetresponse sequence data.
 8. The method of claim 7, wherein minimizing theerror function comprises minimizing a norm of the warping function. 9.The method of claim 1, wherein the variations in one or more stackparameters indicate spatial variations over the substrate.
 10. Themethod of claim 8, wherein the one or more stack parameters comprise atleast one layer height of a layer comprised in the target.
 11. Themethod of claim 8, wherein the one or more stack parameters comprise aplurality of layer heights of different layers comprised in the target.12. The method of claim 1, wherein the measured target response sequencedata relates to one or more dark-field measurements of the one or moretargets.
 13. The method of claim 1, wherein each of the one or moretargets comprises at least two sub-targets, each sub-target having adifferent imposed overlay bias.
 14. The method of claim 1, wherein thevariations of the plurality of measurement profiles comprises variationsof an intensity distribution of the measurement radiation of theplurality of measurement profiles.
 15. The method of claim 1, whereinthe variations of the plurality of measurement profiles comprisesvariations of a wavelength of the measurement radiation of the pluralityof measurement profiles.
 16. The method of claim 1, wherein thevariations of the plurality of measurement profiles comprises variationsof a polarization of the measurement radiation of the plurality ofmeasurement profiles.
 17. The method of claim 1, wherein the variationsof the plurality of measurement profiles comprises variations of anillumination angle relative to the substrate of the measurementradiation of the plurality of measurement profiles.
 18. The method ofclaim 1, wherein the measured target response sequence data and thereference target response sequence data each comprise an intensitymetric as a function of measurement radiation wavelength, the intensitymetric describing an intensity difference between a pair of non-zerothdiffraction orders.
 19. A metrology apparatus comprising: anillumination system configured to illuminate one or more targets formedon a substrate by a lithographic process with measurement radiationcomprising a plurality of measurement profiles; a detection systemconfigured to detect scattered radiation arising from illumination ofsaid one or more targets; and a processor operable to: derive measuredtarget response sequence data from the detected scattered radiation,wherein the measured target response sequence data describes a variationof a measurement response of the one or more targets in response tovariations of the plurality of measurement profiles, compare themeasured target response sequence data to reference target responsesequence data relating to a measurement response of the one or moretargets as designed to the measurement radiation, wherein the referencetarget response sequence data describes an optimal measurement responseof the one or more targets in response to a designed plurality ofmeasurement profiles without un-designed variation, and determine valuesfor variations in one or more stack parameters of the one or moretargets from the measured target response sequence data based on thecomparison.
 20. A non-transitory computer program product comprisingmachine readable instructions which, when run on a suitable processor ofa lithographic system, cause the processor to perform a method ofmetrology in a metrology apparatus for monitoring a measurement process,the method comprising: obtaining measured target response sequence datarelating to a measurement response of one or more targets formed on asubstrate by a lithographic process to measurement radiation comprisinga plurality of measurement profiles, wherein the measured targetresponse sequence data describes a variation of the measurement responseof the one or more targets in response to variations of the plurality ofmeasurement profiles; obtaining reference target response sequence datarelating to a measurement response of the one or more targets asdesigned to the measurement radiation, wherein the reference targetresponse sequence data describes an optimal measurement response of theone or more targets in response to a designed plurality of measurementprofiles without un-designed variation; comparing the measured targetresponse sequence data and the reference target response sequence data;and determining values for variations in one or more stack parameters ofthe one or more targets from the measured target response sequence databased on the comparison.